= Underdominant allele-frequency dynamics
{title2=$\dot u=u(1-u)[(a+b)u-b]$}
With <homozygote> fitness advantages $a,b>0$ over the <heterozygote>, the <allele frequency> $u$ of the first <allele> has an unstable threshold $b/(a+b)$. Below it that <allele> decreases; above it it increases. In a discrete viability-selection generation the same numerator gives $u'-u$ after division by <mean> <genotype fitness>. Equal <homozygote> advantages give $\dot u=su(1-u)(2u-1)$, with stable endpoints and unstable midpoint.
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